This invention relates to processes for coloring metal flakes of the type employed as pigments in paints or like compositions, and to the colored flake products so produced. In an important specific sense, to which detailed reference will be made below for purposes of illustration, the invention is particularly concerned with the coloring of aluminum flakes. The term "aluminum" is used herein to refer to aluminum metal and aluminum-based alloys.
In present-day commercial practice, aluminum flake pigment is made by grinding (e.g., ball milling) aluminum foil or aluminum powder in an organic vehicle, and in the presence of a small amount of a milling lubricant such as a fatty acid, to produce flake-shaped aluminum particles typically having a thickness of about 1 to 2 microns and a diameter of about 10 to 100 microns. The pigment is usually sold as a paste of these flakes in mineral oil.
Aluminum flake pigments are widely employed in organic solvent-based paints, for example paints for coating furniture, to impart metallic luster to the produced coatings or finishes. Although the natural metal color of the flakes is satisfactory for some applications, it is frequently desired to produce a finish of a different color. In such cases it has heretofore been customary to use organic pigments; but the colors thus provided tend to fade away over time, especially after prolonged exposure to sunlight.
For this reason, it would be desirable to color aluminum (and other metal) flakes by applying an inorganic coating thereto. Colors produced by inorganic coatings are superior to organic colors in stability, remaining satisfactorily unfaded for much longer periods under equivalent exposure conditions. The coating of metal flakes with inorganic colors, however, is attended with other difficulties, such as agglomeration, adhesion, and adverse chemical reactions.
U.S. Pat. No. 4,978,394 describes the coating of aluminum flakes with titanium compounds, including TiO.sub.2, by decomposition of TiCl.sub.4 in the presence of water vapor and nitrogen in a fluidized bed of the flakes, to impart interference colors to the flakes. Control of the produced color by variation in coating thickness and/or by other, subsequent treatments (including doping or overcoating with oxides of other metals, such as iron or chromium) is also described. The procedure appears to require a period of several hours and to be capable of performance only on a batch basis.